1. Field of the Invention
This invention relates to a disk drive having at least one rotating memory disk. More particularly, this invention relates to a clamp and method of clamping one or more rotating memory disks to a hub and spindle to form a disk stack in which the disk distortion is minimized or substantially eliminated.
2. Description of the Prior Art
Many disk drives include more than one disk. The disks are typically stacked on a hub. The hub has a lip which typically is on one end of the hub and contacts the surface of the first disk near the inner diameter of the disk. Between each disk is a spacer. The disks and spacers are generally referred to as the disk stack. The term disk stack also applies to a disk drive having only one disk and no spacer. The disk clamp provides a compressive load on the disk stack to hold the disks in place. The compressive load acts on the inner diameter of the disk or disks in the disk stack and is in a direction which is parallel to the axis of the hub. Many refer to this compressive load as an axial load since it acts in the axial direction.
Clamps for disk stacks come in a variety of configurations. In the past, some disk clamps have provided the axial load using screws that are passed through a circular plate into tapped openings in the hub. This configuration had several problems. The circular plate and screws add height to the disk stack. The move to make smaller, shorter disk drives made this arrangement undesirable. In addition, as the disks themselves became thinner, the individual screws produced localized stresses which had the effect of causing the disks to distort at the inner diameter. The disk actually became wavy at the inner diameter. Many in the industry referred to the resulting distorted shape as "potato chipping" because of the resemblance of the disk's shape to a potato chip. Usually, there are as many lobes in the potato chip as there are screws. In many disk drives the transducer is passed over the disk. Ideally, the height between the transducer and disk should be uniform. When potato chipping occurs on a disk, the fly height varies and the data channel must compensate for the variation in the signal from the transducer.
Other clamp configurations include a disk or bell-shaped part that acts as a spring. A screw or screws are passed through openings in the center of the disk or bell-shaped part and into a tapped opening in the hub. This design also has problems. Providing a hub with enough material for a tapped opening requires height. In addition, attaching the screw or screws at the center of the hub causes the disk or bell-shaped part to flatten as the screw is tightened. The edges of the disk or bell-shaped part which contact the disk during tightening move across the surface of the disk in a radially outward direction. The movement of the disk clamp with respect to the disk causes distortion which makes the disk become conical in shape. Many of these disk clamps had square edged perimeters which contacted the disk. Other configurations of the clamps have rounded edges which contact the disk. Both of these configurations produce radial loads on the disk as the clamp moves with respect to the disk. The result is coning of the disk. Others refer to this distortion problem as disk droop.
Another disk clamp is a heat-shrink ring which is attached to the top of the hub. No screws are used. A ring is heated so that it expands and the inner diameter is greater than the outer diameter of the hub. A tool is used to transfer the heated ring to the top of the disk stack and to apply a clamping force to the heated ring. The clamping force is maintained on the ring as it cools. This type of disk clamp also results in coning of the disk. As the clamp load is applied to the clamp the edge of the clamp ring moves radially outward across the surface of the disk. When the heat-shrink ring cools, the edge of the ring moves inward across the disk surface near the inner diameter. The inward motion is smaller in magnitude than the outward motion and the resulting relative displacement between the disk and the ring results in a radial load on the surface of the disk that cones the disk.
Any distortion due to clamping is undesirable. The slider and transducer do not maintain a constant flyheight in the presence of the potato chip type disk distortion. A valley causes the slider and attached transducer to fly low while a hill causes the slider and attached transducer to fly high. In the presence of a coning type disk distortion, the flyheight is too high on one side of the disk and too low on the other side of the disk. The flyheight may be constant but will vary from the nominal, designed flyheight.
Several clamps have addressed the problem of disk distortion. In Scheffel U.S. Pat. No. 4,918,545, the clamp shown clamps the disk pack without placing a radial load on the disks in the disk stack. The clamp requires an additional spacer ring on the top of the disk stack about which a portion of the rim 25 rotates. The spacer ring 21 provides a pivot point about which the rim of the clamp rotates. The clamping force is provided by placing a screw through the clamp and into a tapped opening in the hub. The additional spacer ring and the screw into the hub each result in additional height which is undesirable. A clamping arrangement taught in IBM Technical Disclosure Bulletin, vol. 26, No. 10B, March, 1984, pp. 5444-5446, entitled "Zero Droop Disk Clamping Assembly", does not eliminate placement of a radial load on the disks but rather shifts the geometry of the hub to accommodate it.
As a result, there is a need for a disk clamp and clamping method which minimizes disk distortion while also minimizing the amount of height needed to accommodate the clamp.